BIG PHYSICS, BIG QUESTIONS –

Wiping out top predators messes up the climate

FLATTENED forests and melted permafrost boost greenhouse gas emissions – and now it seems the plight of nature’s top predators, too, will speed up the planet’s lurch towards a hotter climate.

Mass extinctions of the big beasts of the jungles, grasslands and oceans could already be adding to emissions, according to a new study based on fieldwork in aquatic ecosystems. Equally, saving such species could keep greenhouse gases locked away, raising the possibility of conservationists claiming carbon credits for protecting charismatic predators.

“It looks like predators in many types of ecosystems can play a big role in global climate change,” says Trisha Atwood of the University of British Columbia in Vancouver, Canada, who led the study.

The dramatic impact on ecosystems of losing top predators is well known. In the ensuing “trophic cascade”, the animals they feast on proliferate, which in turn puts pressure on their prey, and so on. In this way, changes at the top of a food chain destabilise populations right the way down. And since plants – important carbon sinks – sit at the base of most food chains, any change at the top could affect the overall carbon cycle.

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Atwood and her team tested the idea in Canada and Costa Rica by temporarily removing fish and insect top predators from ponds, streams and tiny wet ecosystems associated with bromeliad plants. They then logged the impact on the local biomass, including its rate of decomposition – a process which produces emissions. They also monitored how much carbon dioxide was released into the atmosphere.

Global climate models do not yet take such impacts into account. Atwood says the effects could be major as freshwater emissions may be on a par with the influence of deforestation, which is thought to account for around 15 per cent of human-caused CO2 emissions.

The effect will not always be to increase emissions, however&colon; they may sometimes fall when top predators disappear, according to modelling Atwood carried out alongside the experiments. Either way, she says, the work shows that “something seemingly unrelated, like fishing all the trout from a pond or removing sharks from the ocean, could have big consequences for greenhouse-gas dynamics”.

Something like removing sharks from the ocean could have a big impact on greenhouse gases

Other studies hint at similar effects. Christopher Wilmers of the University of California, Santa Cruz, and colleagues showed last year that vanishing sea otters are linked with higher emissions from North American coastlines (Frontiers in Ecology and the Environment, doi.org/khz). With no otters to eat them, sea urchins thrive and gorge on kelp forests – often called the “rainforests of the oceans” – resulting in major CO2 releases.